import java.util.Scanner;

public class HomeWorkDemo04 {
    public static void main(String[] args) {
        double sum = 0.0;
        for (double i = 1; i <= 20; i++) {
            sum += 1 / (i * i);// sum = sum+ 1/i*1

        }
        System.out.println(sum);


    }

}
